The present invention relates to hydraulic or oil pressure control systems which are used in oil circuits for driving actuators built in machines designed for construction work and, more specifically, to an oil pressure control system in which a flow control valve is provided in an oil input circuit of an actuator to control the flow control valve under the control of a pilot valve.
A prior art oil pressure control system of the type referred to is disclosed in U.S. Pat. No. 4,535,809 in which, as shown in FIG. 1, flow control valves `g` are provided in oil input circuits `c1` and `c2` connecting an oil pressure pump `a` and an actuator `b` and also in oil output circuits `e1` and `e2` connecting the actuator `b` and a tank `d` to control the inflow rate of the actuator `b` according to the opening of the respective pilot valves `f`.
FIG. 2 shows a particular arrangement of the pilot valve `f` and the flow control valve `g`. More specifically, input and output ports `h` and `i` of the flow control valve `g` are opened and closed by a poppet `j`. The poppet `j` is provided with a throttling slit `l` to controllably throttle oil under pressure flowing into the input port `h` and send it through the slit to a back pressure chamber `k`. The pilot valve `f` is inserted in a pilot circuit `m` connected between the back pressure chamber `k` and the output port `i`.
In the operation of foregoing prior art example, when the pilot valve `f` is throttled to be opened by a predetermined amount while the input port `h` receives oil under pressure, pilot oil flows through the pilot circuit `m` at a flow rate corresponding to the opening of the pilot valve `f`. This pilot oil causes the development of a pressure difference between the input port `h` and the back pressure chamber `k`, whereby the poppet `j` is opened by an amount corresponding to the pressure difference so that a predetermined amount of pressurized oil flows from the input port `h` to the output port `i`. The gradual opening of the poppet `j` causes the opened amount of the throttling slit `l` to be correspondingly increased to gradually increase the rate of the pilot oil flowing from the input port `h` to the back pressure chamber `k` and gradually decrease the pressure difference between the input port `h` and the back pressure chamber `k`. The movement of the poppet `j` is stopped as soon as the pressure difference reaches zero. In this way, the flow rate of oil under pressure flowing from the input port `h` to the output port `i` is controlled not by the pressure of the input port `h` but by the opening of the pilot valve `f`.
Shown in FIG. 3 is another prior art oil pressure control system in which a flow control valve `g` has a fixed orifice `l1` provided between a poppet `j` and an input port `h` to develop a pressure difference therebetween as well as a variable throttle `l2` provided between a back pressure chamber `k` and another output port of the valve `g` leading to the pilot valve `f` to decrease the opening of the valve `g` as the poppet `j` moves upwards. In this control system, when the pilot valve `f` is operated to increase the throttle opening area, a pressure Pp at the entrance side of the pilot valve `f` is reduced and a pressure P.sub.B in the back pressure chamber `k` of the flow control valve `g` is lowered. This cause a pressure difference to be developed between both ends of the fixed orifice `l1` of the poppet `j` so that this pressure difference causes upward movement of the poppet `j`, which results in that the input port `h` communicates with the output port. As the poppet `j` moves upwards, the opening area of the variable throttle is gradually reduced and correspondingly the pressure of the back pressure chamber `k` is increased until the poppet `j` stops. In other words, in the oil pressure control system, the poppet `j` is located at a desired position by decreasing an equivalent throttle opening area corresponding to a sum of the throttle opening area of the variable throttle `l2` and the throttle opening area of the pilot valve `f` to increase the pressure P.sub.B.
There is shown in FIG. 4, an oil pressure control system as yet another prior art, wherein, in a flow control valve `g`, a metering pin `r` is inserted into an axially-extended bore made in a poppet `j` so that the metering pin `r` and a slit provided in the poppet `j` form a variable throttle S between an input port `h` and a back pressure chamber `k`. In the operation of this oil pressure control system, when the pilot valve `f` is actuated to lower the pressure of the back pressure chamber `k`, a pressure difference takes place between upper and lower pressure acting surfaces of the poppet `j` to move up the poppet `j` and communicate the input port `h` with the output port `i`. As the poppet is moved up, the opening of the variable throttle S increases and the pilot oil rate flowing from the input port `h` to the back pressure chamber `k` increases, whereby a pressure difference between the input port `h` and back pressure chamber `k` is gradually reduced to zero, at which time the movement of the poppet `j` is stopped.
In the foregoing prior-art pressure control systems, when the pilot valve `f` is operated to provide such a pilot flow as shown by a dotted line in FIG. 5, as explained above, a pressure difference between the input port `h` and back pressure chamber `k` of the flow control valve `g` causes the poppet `j` to be opened so that oil under pressure flows from the input port `h` to the output port `i`, thus increasing the pressure of the output port `i`. The increased pressure of the output port `i` is applied to the pressure receiving surface of the poppet `j` provided on the side of the output port `i`. For this reason, an increase in the pressure of the output port `i` causes the poppet `j` to be momentarily opened to an extent larger than a predetermined amount. Therefore, the prior art systems have had such a problem that a curve indicative of the main flow rate flowing through the flow control valve `g` has a projected part in its initial stage as shown by a solid line in FIG. 5, which means that the initial stage operation of the flow control valve `g` causes momentary, abrupt operation of the actuator associated with the valve.